Differential expression of iodothyronine deiodinases in chicken tissues during the last week of embryonic development

In the current study, the authors examined the type 1 (D1), type 2 (D2), and type 3 deiodinase (D3) activity and mRNA expression patterns in thyroid, lung, brain, pituitary, heart, liver, spleen, gonads, skin, muscle, intestine, Fabricius' bursa, and kidney during the last week of chicken embryonic development and the first 2 days posthatch. The D3 was the most widely expressed, occurring in all examined tissues. Also, the D1 knows a widespread distribution, although no D1 activity or mRNA expression could be detected in the brain, the thyroid, the muscle, and the skin. In contrast, the D2 has a much more restricted expression pattern, since the brain is the only organ where, prior to hatching, both in vitro D2 activity and D2 mRNA expression can be detected. Taken together, these results demonstrate that during the last week of chicken embryonic development, the majority of tissues express D3, together with either D1 or D2, indicating that each tissue possesses the necessary tools to regulate local thyroid hormone levels at least partly independent from T(3) and T(4) levels in plasma. In addition, the deiodinase expression data could be correlated to certain thyroid hormone dependent tissue-specific developmental events. This strongly suggests that in birds, as in mammals and amphibians, the correct spatial and temporal expression of iodothyronine deiodinases are essential for normal embryonic development.